\begin{tikzpicture}[>=Stealth, scale=0.5,
    every node/.style={fill=white, inner sep=1pt},
]
    \draw [name path=x axis, ->] (-2.5, 0) -- (3.5, 0) node[below=0.2em] {$x$};
    \draw [->] (0, -3) -- (0, 3) node[left=0.2em]  {$y$};
    \draw (0, 0) node [below left=0.1em] {\small $O$};

    \pgfmathsetmacro{\a}{1}
    \pgfmathsetmacro{\b}{-1}
    \pgfmathsetmacro{\c}{-2}

    \draw[name path=parabola, domain=-1.5:2.5, samples=50] plot (\x, {\a*(\x)^2 + \b*\x + \c});
    \fill [black, name intersections={of=x axis and parabola}]
        (intersection-1) circle (0.1) node[above left, xshift=-0.3em] {\small $x_1$}
        (intersection-2) circle (0.1) node[above right, xshift=0.3em] {\small $x_2$};
\end{tikzpicture}

